Inbound NAT allows you to open up TCP and/or UDP ports or port ranges to hosts on networks protected by m0n0wall. You may need to open ports to allow certain NAT-unfriendly applications and protocols to function properly. Also if you run any services or applications that require inbound connections to a machine on your internal network, you will need inbound NAT.
Inbound traffic is incoming data that arrivs on the selected m0n0wall NAT interface that has not already travelled througn th m0n0wall itself. For example, inbound traffic on the WAN interface coming directly from the Internet can have inbound rules applied to it but traffic from the LAN network that goes through the WAN interface cannot have inbound rules applied because that traffic had to pass through the m0n0wall to arrive at the WAN interface.
It is not possible to access NATed services using the WAN IP address from within LAN (or an optional network). Only external traffic incoming on the selected interface will have Inbound NAT rules applied to it.